Abstract The anti-apoptotic protein cellular FLICE-like inhibitory protein (cFLIP) has been shown to play an important role in survival of T and B lymphocytes; however, the function of cFLIP in other immune cells is unknown. To examine the role of cFLIP in myeloid cell survival and function, we generated mice expressing a loxP-flanked cFLIP allele in the presence of Lysozyme M-Cre (cFLIPf/f Lysm-Cre). cFLIPf/f Lysm-Cre mice lack splenic marginal zone macrophages and thioglycollate-elicited peritoneal macrophages, while inflammatory monocyte and neutrophil numbers are increased. Using this model, we find that loss of cFLIP expression in macrophages and neutrophils results in a severely inflammatory phenotype, with cFLIPf/f Lysm-Cre mice displaying splenomegaly, decreased body weight, profound neutrophilia, increased production of IL-1β and GCSF, and extramedullary hematopoiesis. These defects are secondary to the loss of macrophage populations, as mixed bone marrow chimeric mice do not develop inflammation. Together, these data demonstrate a requirement for cFLIP expression for macrophage survival along with a requirement for mature macrophage populations to maintain neutrophil homeostasis in vivo. This work is supported by an NSF Graduate Research Fellowship and NIH grants.